SNES and Sega Genesis cartridges are small computers, not merely storage cards. A cartridge combines read-only game memory, address-decoding logic and, when a game supports saves, writable static RAM (SRAM). A coin cell keeps that SRAM powered after the console is switched off. When the cell dies, the game can still boot from ROM, but its saved data may disappear.
This guide explains the cartridge hardware, how saves survive a power-off, why SNES and Genesis boards differ, and how to replace a battery while giving your save the best chance of surviving.
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What is inside a cartridge?
An original cartridge contains a printed circuit board (PCB) with one or more memory chips and support components. The console connects to the PCB through the edge connector and places addresses, data and control signals on that interface. Logic on the cartridge decides which chip responds to each address.
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The game itself is stored in mask ROM or a similar non-volatile ROM chip. It holds executable code, graphics, sound data, maps and other assets. ROM can be read when the console is powered, but the game normally cannot rewrite it.
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SRAM: the writable part
A save-capable cartridge adds static RAM. SRAM is fast and does not need a refresh cycle, but it is volatile: its bits are lost when power falls below the level needed to maintain them. The game writes save records into particular SRAM addresses and reads those same addresses at the next boot.
SNESdev uses the .SRM extension for a dump of battery-backed cartridge save RAM. The SNES ROM header records the amount of save RAM when it exists, and its chipset field indicates whether a board includes extra RAM, a battery and/or a coprocessor.
Decoders, mappers and other logic
Address-decoding logic maps the console’s address space onto the physical ROM and RAM chips. A common SNES part is the MAD-1 decoder. More elaborate boards add a mapper or a coprocessor for special graphics, mathematics or compression tasks. These chips do not replace the basic ROM-plus-SRAM model; they expand how the console reaches the data.
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LoROM and HiROM mapping
SNES software uses several address-mapping schemes. In LoROM, the cartridge presents ROM in 32 KB banks. HiROM presents 64 KB banks. Extended mappings support cartridges that exceed the simpler layouts or include additional hardware. The mapping determines which physical ROM location a CPU address selects; it does not change the underlying save principle.
Game sizes and typical save RAM
Epilogue’s 2026 cartridge guide gives a range of 2 megabits (256 KB) to 48 megabits (6 MB) for SNES game chips. Those figures describe ROM capacity, not the size of a save. The same guide identifies 8 KB as a typical SRAM size, with larger amounts used by some games. A title’s header and board design determine the actual amount.
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Coprocessor boards
Some SNES cartridges add chips such as Super FX, SA-1, DSP or decompression hardware. These boards can have unusual layouts and additional memory. A battery-backed save still works the same way: the game stores bytes in SRAM, and a standby supply keeps the SRAM contents alive while the console is off.
How Sega Genesis cartridges organize memory
Genesis cartridges follow the same broad architecture but use title-specific boards and address ranges. A battery-save board normally contains mask ROM for the game, a static-RAM chip for writable data, a small support IC and a coin cell with retention circuitry.
ConsoleArtisan’s documented EA board, for example, shows a mask ROM, a GM76C88 SRAM, a support IC and a Seizaiken CR2032 3 V cell. Other Genesis boards may use different SRAM chips, address ranges or battery mounting methods. Therefore, do not infer the exact layout of one cartridge from another.
Some Genesis games have no writable memory at all and use passwords or another progress system. A cartridge that lacks SRAM and a battery cannot be made into a battery-save game merely by adding a cell; the game software and board wiring must support saves.
What happens when you save and switch the console off?
- The game writes a record. The program sends save data, checksums or other metadata to designated SRAM addresses.
- Console power runs the cartridge. While the SNES or Genesis is on, the cartridge receives operating power through the edge connector. The SRAM is fully powered and can be read or written.
- The console is switched off. Operating power disappears. A power-switching circuit changes the SRAM to a low-power standby supply.
- The coin cell maintains the bits. The battery supplies the tiny standby current needed to keep SRAM contents intact.
- The next boot reads the record. The game reads the same addresses, verifies its save structure and loads the progress if it is valid.
The Snes9x project describes this real-cartridge model succinctly: “The game cartridge contains a battery-backed RAM, known as SRAM, and your save data remain in this SRAM until the battery shutoff.” That statement dates from 2011, but it describes the electrical arrangement used by original battery-save cartridges.
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Why a game can boot after its battery dies
ROM and SRAM have separate jobs. The console can execute the game entirely from ROM, so a dead battery does not normally prevent the title screen from appearing. The battery only preserves SRAM while the system is off.
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Once the cell can no longer maintain standby voltage, SRAM contents become undefined or are lost. The program may report an empty save, show corrupted entries or behave as if the cartridge has never been played. Replacing the battery restores the standby supply, but it cannot recover bytes that were already lost. That is why a game that still boots can nevertheless have a failed save system.
Which cartridges use batteries?
There is no reliable published percentage for all SNES or Genesis releases that used batteries. The safe rule is to identify the individual title and PCB.
- Battery-backed SRAM: The common design for games with persistent saves on both systems.
- No save hardware: Some games rely on passwords or have no progress storage.
- Non-battery methods: Later or unusual boards may use another non-volatile memory technology, so the presence of a save menu alone does not prove that a CR2032 is installed.
Look for a coin cell, a battery holder or a welded metal-tab cell inside the shell. Do not assume a board has a battery because another game from the same console does.
SNES and Genesis cartridge differences
| Feature | SNES | Sega Genesis |
|---|---|---|
| Typical save medium | Battery-backed SRAM when the title supports saves; some games use passwords or have no save. | Battery-backed SRAM when the title supports saves; some games use passwords or have no save. |
| ROM mapping | LoROM (32 KB banks), HiROM (64 KB banks) and extended mappings. | Layouts and SRAM address ranges vary by title and board. |
| Common support hardware | MAD-1 decoder and, on selected games, Super FX, SA-1, DSP or decompression chips. | Title-specific mappers or other support logic, alongside the ROM and SRAM. |
| Typical battery | CR2032 cells are common; tabs, holders and placement vary. | CR2032 cells are common; tabs, holders and placement vary. |
| Known capacity figures | Epilogue lists 2 megabits (256 KB) to 48 megabits (6 MB) for game chips and about 8 KB typical SRAM, with larger SRAM on some titles. | No single capacity range applies to every board; inspect the specific PCB. |
The important distinction is not the brand of console but the board in your hands. Two cartridges that look similar externally can have different memory maps, battery connections and save sizes.
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How to replace an SNES or Genesis save battery safely
Battery replacement is a preservation job, not just a parts swap. The highest-risk moment is removing the old cell before its SRAM contents have been copied.
1. Back up the save first
Use a compatible cartridge reader or dumper to read the SRAM and save a backup file before opening the shell. Keep the original dump in more than one location. A backup is the only dependable recovery method if the old cell is already weak or the replacement goes wrong.
2. Gather the correct tools and replacement
- A replacement CR2032 3 V cell with the same tab or holder arrangement as the original board.
- A compatible cartridge reader for SRAM backup and later restoration, when available.
- A suitable screwdriver and an SNES/Super Famicom 3.8 mm security bit. Epilogue specifies the 3.8 mm bit for opening those shells.
- Eye protection and a temperature-controlled soldering setup if the cell is soldered. Avoid shorting the cell or overheating nearby pads.
3. Open the shell and identify the mounting method
Remove the screws carefully and lift the shell without flexing the PCB. Photograph the board before touching anything. Genesis cells may be soldered directly or held by a metal tab with friction. SNES boards likewise differ in holder, tab orientation and polarity marking.
4. Record polarity and board orientation
Mark the positive and negative sides and compare them with the PCB markings. A reversed cell can prevent retention and may damage the circuit. Match the original physical arrangement rather than relying on a generic cartridge photograph.
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5. Replace the cell
Remove the old battery using the method appropriate to its holder or solder tabs. Install a cell with matching tabs or fit the correct holder; do not solder directly to a bare consumer coin cell unless it is designed for that purpose. Keep the replacement from touching adjacent traces or components.
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6. Test and restore
Reassemble enough of the shell to protect the PCB, start the game and confirm that it boots. If the reader supports writing, restore the saved SRAM backup and verify that the in-game save appears. Create a new test save, power the console off, wait briefly and boot again to confirm retention.
If you cannot back up SRAM and the cartridge contains a soldered cell, assume the save may be lost during removal. A professional technician or an experienced retro-hardware service can reduce the risk, but no replacement method can guarantee recovery from already-corrupted SRAM.
Troubleshooting save problems
The game boots but every save is gone
- Check whether the cartridge actually contains SRAM and a battery; the title may use passwords instead.
- Inspect the cell for leakage, corrosion, a broken tab or reversed polarity.
- Measure the replacement and inspect solder joints if the cell was recently changed.
- Restore a known-good SRAM backup to distinguish a bad save file from a power-retention fault.
The game reports a corrupt save
Corruption can result from a failing cell, interrupted writes, dirty contacts or damaged SRAM. Clean the edge connector with an appropriate electronics-cleaning method, test with a stable replacement cell and compare the SRAM dump with an earlier backup.
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The replacement battery does not last
A fresh CR2032 should not be rapidly depleted by normal standby operation. Check for a short, incorrect battery type, damaged power-switching components or a board that is drawing excessive current. Do not assume the cell itself is defective until polarity and solder bridges have been inspected.
The cartridge does not boot after service
Reseat the PCB, inspect for lifted pads or solder bridges and clean the edge connector. Confirm that no component was installed backward and that the shell is not pressing on the board. If the game still fails, reinstall the original board orientation and seek board-level diagnosis rather than repeatedly powering it.
Preserving cartridges for the long term
Back up important saves before a battery reaches the end of its life, especially for cartridges that have been stored for decades. Keep cartridges dry, avoid extreme heat and do not leave corroded cells in place. Store backup files with the title and region identified, since SRAM formats and memory sizes can differ between releases.
A battery replacement preserves the electrical conditions for SRAM; it does not repair damaged traces, failed SRAM, corrosion or a defective decoder. Treat the battery, holder, PCB and save backup as four separate parts of the preservation plan.
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Key points to remember
- ROM runs the game; battery-backed SRAM stores writable progress.
- The coin cell powers SRAM only while the console is off, so a dead battery usually causes lost saves rather than a failure to boot.
- SNES mapping may be LoROM, HiROM or an extended variant, while Genesis memory layouts vary by title.
- CR2032 cells are common, but tab style, holder design and polarity are board-specific.
- Dump the save before opening the cartridge, install matching hardware and verify retention afterward.
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